Convert large particle system(s) to one object efficiently
Particle Instance modifier


  When a ParticleInstance modifier is added to an object, that object will be used as a particle shape on an object which has a particle system associated with it.



Instead of setting the leaf object in the render settings of your tree's particle system.
Use the "Particle Instance" modifier on the leaf itself.


This has the advantage that it can be used multiple times for multiple particles systems.